ramips: add support for Cudy R700

ramips: add support for Cudy R700

This is the same hardware as the Cudy WR2100 that's
had support for some time now, just without the WLAN
hardware.

This PR is mostly copied from the commit that added
support for the WR2100, here: 3501db9

Specifications:
SoC: MT7621
CPU: 880 MHz
Flash: 16 MiB
RAM: 128 MiB
Ethernet: 5x Gbit ports

Installation:
There are two known options:

The Luci-based UI.
Press and hold the reset button during power up.
The router will request 'recovery.bin' from a TFTP server at
192.168.1.88.

Both options require a signed firmware binary.
A signed firmware can be found in GitHub PR #18532.

R4 & R5 need to be shorted (0-100Ω) for the UART to work.

Link: https://github.com/openwrt/openwrt/pull/18532
Signed-off-by: David DeGraw <degraw@fastmail.com>
(cherry picked from commit 75403dd1d05bea1b65349ebb9f8b7deb40b9c802)
Link: https://github.com/openwrt/openwrt/pull/21068
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>